Microwave detector network



Jan. 3, 1950 T. M. GLUYAS, JR l 2,493,066

MICROWAVE DETECTOR NETWORK Filed Ju1y'24, 1945 1i/75%' l maffia nrrf/vz/mf/z//f 1f if Patented Jan. 3, 1950 MICROWAVE DETECTOR NETWORK Thomas M. Gluyas, Jr.,

Westmont, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application July 24, 1945, Serial No. 606,806

14 Claims. l

This invention relates generally to microwave communication systems and more particularly to an improved microwave detector network for superheterodyne receivers including automaticfrequency-control circuits.

Eilicient centimeter wave receivers have been developed utilizing crystal detectors of the silicon-tungsten or other known types coupled to an input waveguide system which is connected to a microwave antenna. The received signals are modulated by a source of local oscillations to provide an intermediate frequency signal, and the received signals and local oscillations also are applied to a second crystal to derive potentials for automatic frequency control of the receiver oscillator tuning. The instant invention is an improvement upon such microwave superheterodyne converter and automatic-frequency-control circuits for a microwave transmitter-receiver system for radar or other pulse transmission purposes.

The instant system includes a pulse transmitter, such as a magnetron, coupled through a transmission waveguide to a common transmitting and receiving antenna. Branch waveguides connected to the main transmission waveguide include ionic discharge type cavity resonator T-R and anti-T-R boxes (selective transmitreceive switching devices). Such T-R. boxes are well known in such systems and permit alternate isolation of the transmitter and the receiver from the antenna system in response to the transmitted microwave energy. When the transmitter is inoperative (for example, during the time intervals between transmitted pulses), the T-R box permits received signals to be applied tothe signal detector and the anti-T-R box isolates the transmitter and its connecting waveguide from the antenna system. The T-R boxes may comprise adjustably tuned cavity resonators having an ionizable gap disposed to break down when an intense field is established within the resonator due to transmitted pulses. The ionizable gap interrupts microwave transmission through a waveguide system coupled through, or to, the T-R box, or may be employed to change 'ie tuning of a waveguide system connected thereto.

The novel features of the instant invention comprise a unique arrangement of the elements comprising the T-R box, receiver rst detector, local oscillator injection network, and automatic freauency control detector. Previous systems have had the disadvantage that objectionable reaction occurredbetween the rst signal deter..

tor and the automatic-frequency-control detector and that objectionable loading of either detector or the local oscillator was provided by the adjacent T-R box or automatic-frequency-control signal input circuit. Previous systems have had another disadvantage that the local oscillator placed objectionable loading on the signal detector with consequent loss of signal energy and deterioration ofsignal-to-noise ratio.

The present system includes a signal detectorautomatic frequency control crystaly network comprising a waveguide coupled through the tunable T-R box to the antenna-transmitter waveguide system. The signal detector comprises a crystal matched with respect to impedance to the detector waveguide, and spaced one and three-eighths wavelengths from the tunable T-R box. A tunable aperture is disposed at a distance of .482 wavelength from the signal crystal, intermediate the crystal and the T-R box, whereby the tunable T-R box and the tunable aperture comprise a double stub tuner which will match with respect to impedance all conventional types of microwave crystal detectors.

The automatic-frequency-control crystal is spaced a distance of approximately two wavelengths further along the detector` waveguide, and the local oscillator signal is injected into a non-resonant cavity disposed substantially midway between the two detectors. The non-resonant cavity is formed by two tuned aperturedevices located about one-half wavelength apart and including two conductive stubs extending transversely therein in planes parallel to' the electric eld. The oscillator frequency is Selected to be higher than the received signal frequency by an amount equal to the desired intermediate frequency.

Since the T-R box is tuned to a lower frequency than the local oscillator frequency, it is reactive, and tunes the one and three-eighths wavelengths line between it and the signal detector to provide a high impedance across the signal crystal at the oscillator frequency. This arrangement prevents loading of the oscillator output energy by the T-R box as well as minimizing undesirable shifting or pulling of the oscillator frequency, and allows the oscillator to be tuned thru a range of frequencies without changing the coupling to the signal detector, an essential feature in sets employing A. F. C. This arrangement is especially desirable since it is essential that the signal detector must not be shunted by excessive conductance or susceptance 55 reilected from the T-R box cavity. The tuning of the two tuned aperture devices forming the boundaries of the non-resonant cavity into which the local oscillations are injected provides the desired degree of coupling to the signal detector and to the automatic-frequency-control detector. These apertures are normally considerably removed from resonance and are low impedance circuits .across the @guide at signal frequency. They are -located an oddnumber of quarter wavelengths from the detector and therefore reflect a high impedance across the detector so that signal energy is not lost by passing. ron -to the local oscillator cavity or through to the A. F. C. detector.

The local oscillator may comprise, forexample, a conventional tunable microwave tube comprising a tunable cavity resonator :having an-electron source and an electron reilector electrode. The output of the local oscillator may be coupled 'into the non-resonant local oscillator injection cavity through a conventional microwave transmission line or'signal radiating stub inserted within the non-resonant cavity.

Sampling- -of the transmitted microwave energy for the automaticdrequency-control .detector ciraccomplished by means of a small aperture inthe narrow side-of the transmission 'waveguide opening linto-a sampling waveguide disposed at .an angle thereto :in lorder to vobtainia .transverse electric iield component in the sampling waveguide. :singularly disposed .sampling V.waveguide is coupled to a second .sampling waveguide section which includes a tapered absorptive insulating plug providing suihcient microwave attenuation to prevent resonancerin thesampling waveguide line. This avoids selective frequency loading of theA. -F. -C. fdetector 'and 'con-sequent pulling of the local `oscillator Ifi'eonencyi The energy derivcd after ytri'nfisxnissiori vthrough the tapered iinsulsting eattenuator plug is appliedto the remaining Yend of ithe detector waveguide for Aactuating the automatic-ffreouencwcontrol detector.

Thus, .the received ,signals and the aazutomaticfrequency-controlsampling-signalszare applied to .the 'two detectors .from opposite :ends of the .deteotor waveguide system.. Also 'the .local oscilla- .tions are applied separately [to the two .detectors in opposite directions icm ithe center iof :the .detector waveguide system, 'whereby substantially nofreaction .foccurs hetweenithe two :detector circuits tand minimum loading of the local oscillator iszencountered. The :desired lattenuation of the transmitted-signal pulses `ilor the-purpose I.01E factuating the tautoinatic-frenuencyecontrol detector iszprovided .proper selection of the coupling .aperture 4beiwveen ithe sampling waveguide and tnanmissionwvaveguide.

Among the objects of the-.invention are to iprovide an improved .method :of :and means .for re- :netting microwave signals., Another object is to provide an improved `microwave vsuperhetermlyne detector network having 'automatic-frequencyfcontrol achcuits Kassociated therewllm. An addi- :tional objectdsto provide :an 4improved. microwave pulse transmitter-,receiver system having selective fconnections from .1a/pulse transmitter and pulse receiver to 1a :common antenna -system and dncluding means :ior yisolating -.a Asource rof `local mscillations -from input zcircuits having inormally sseriousloadingrcharacteristics. A further object .nf theinvcntionis .to provide an improved meth- .nd mf fandmeans forfselcctively -lnj'ectinglocal Vlosfcillations iin a microwave :superlreterodyne remeiner tothe signal `detector and .to 'an automatic 'jrequencycontroldetecton An additional object is to provide improved methods of and means for preventing overloading and frequency pulling of a local oscillator in a microwave superheterodyne detector network. Another object is to provide an improved method of and means for isolating a signal detector and an automatic-frequency-control detector coupled to a .common local oscillator source in a microwave superheterodyne receiver network. A further object of the invention is to provide an improved microwave superheterodyne detector circuit including a transmit-receive switching device wherein the transmit-receive switching device .comprises a I.portion of the signal detector frequency selectioncircuit. A still further object is to 'provide -an improved method of and means -for suppressing resonance conditions in a normally resonant waveguide system for the detectionof Amicrowave signals.

These and other objects of the invention will be apparent .from the more 'detailed description of the novel .system :amd method illustrated in the accompanying `drawing of which Figure 1 is a plan view of a preferred embodiment thereof and Figure 2 is an elevational view, partly in crosssection, .of said preferred embodiment Vof the invention. Similar' :reference characters are applied-to :similar:elementsthroughout the drawing.

Referring to the. drawing. amain transmission waveguide .I is Vconnectell between a microwave pulse transmitter; such as a magnetron not shown, land an antcnnamOtshoWn. A receiver waveguide 3, including afixed :aperture device 5, branches off `from the transmitter waveguide I and enters a tunable T-:R box 1. At a point spaced. one-quarter Awawelcngth in the ydirection of the transmitter', a second waveguide 9 branches from the fmain transmission waveguide I.. The second waveguide -9 includes a Vsecond aperture. device .I.:I and is terminated in a tunable anti-T-R box I3 of the same general type as the Vbox .'I, .although it should .have a single waveguide connection as inthe -type 724A unit. Both T-'R boxes ,comprisegas-lled cavity resonatorsahavinga-narrow .central gap which breaks down in an `ionic discharge when `the field intensity within the cavity yresonator exceeds predetermined-values.

Both TR boxes discharge-.when a transmitter pulse is propagated alongthe transmitter waveguide eI to the fantenna. When the T-R box 1 is ionized `it effectively prevents transmitted pulses from ,entering the receiver waveguide 3. In the intervals. 'between transmitted pulse energy, the ionicdischarges inlthe'T-'R boxes 'I and I3 cease and `received 4pulses from athefantenna may pass through Ythe :receiver T-R :box '1 to the detector waveguide .I5 which opens into the T-R box. When :nolionic discharge occurs `in the anti-T-R box I3, the pulse transmitter is effectively isolated from the receiver waveguide Aand antenna waveguide systems, since the transmitter waveguide I 'is thus effectively-short-circuited at the ljunction thereof withfthe receiver waveguide 3.

At a distance along the :receiver waveguide I5 spaced :1.'375 wavelengths -at the Aoperating frequency from the :tunable 'If-R box 1 is located a conventional microwave crystal detector I'I which may ,beim-example, of :the silicon-tungsten type. Crystal detectors cf'this-type for microwave irequencies are .described in .the `copending applica- .tion 'of Wendell L. "Carlson, Serial No. 507,755, led:0ctober 26,1943. Atn'pllint on the receiver waveguide lspaced .482 wavelength from the signal crystal .detector Ilrintermediate the detector and the T-R box 1, is located a tuned aperture device I9, the tuning of which may be adjusted by means of a micrometer mechanism 2|. The tuned T-R box 1 and the tuned aperture device |9 comprise a two stub tuner for the signal detector I1.

At another point spaced .75 wavelength further along the receiver waveguide I5 from the signal crystal detector I1 is located a second tuned aperture device 23, the tuning of which may be adjusted by means of a second micrometer control 25. A third tunable aperture device 21 having a third micrometer control 29 is spaced approximately one-half wavelength further along the receiver waveguide, thereby forming a tunable cavity for injection of signals from a local oscillator, not shown. In order that the local oscillator cavity 3| may be non-resonant at the oscillator frequency, it includes a pair of vertical conductive stubs 33 yparallel to the electric field, which effectively prevent resonance in any mode of the cavity.

The local oscillations may be generated by any type of microwave oscillator capable of providing suitable heterodyne signals having an output frequency higher than the received signal frequency. An appropriate local oscillator may comprise, for example, a tuned or tunable microwave tube, such as the type 723A tube. The

loutput transmission line of this tube may be coupled into the oscillator injection cavity 3| by inserting the tube output line through an insulating bushing 35 opening into the oscillator injection cavity. For the purpose of simplifying the drawings, the oscillator tube is not shown.

At a point three-quarters of a wavelength (in terms of mean signal frequency) further removed along the received waveguide I5 from the third tunable aperture device 21, is located a second microwave crystal detector 31 of the same type as the signal detector I1. The second microwave ycrystal detector 31 is utilized for deriving automatic-frequency-controlpotentials for regulating the frequency of the local oscillator signals.

The end of the receiver waveguide I5 adjacent to the automatic-irequency-control crystal 31 opens into a second waveguide 39 which includes a tapered microwave attenuating plug 4| providing approximately 6 decibels attenuation to prevent resonance in the second waveguide 39. The waveguide 39 is coupled at the point `i3 to a sampling waveguide 45, which extends at an angle of approximately 45`therefrom and is coupled through a relatively small aperture 41 into the transmitter waveguide I. transmitted pulses by the coupling aperture 41 is of the order of decibels, thereby providing a signal of proper level for the automatic-frequency-control crystal. The various automaticfrequency-control circuits actuated by the control crystal 31 in response to transmitted pulses and local oscillator output signals do not form a part of the instant invention and are not described in detail herein. However, it should be understood that such control circuits may include circuit components and arrangements well known in the art.

The coupling of the local oscillator signals from the local oscillator injection cavity 3| to the signal detector I1 and to the control detector 31 is separately controlled by means of the adjustable aperture devices 23 and V21, thereby providing a simple and eiective means for regulating and controlling the amount of local oscillator energy coupled to both detectors. i

The attenuation of the :cipal advantages of the instant microwave debox 1 and the iirst aperture device I 9-2I provides ecient coupling vfor received signals from the antenna waveguide to the signal detector The .75k spacing between detector I1 and low impedance aperture 23 prevents loading of Ythe detector at signal frequency by the local oscillator cavity or circuits beyond.

The output of the signal detector I1 is derived from an output terminal 49 which is connected ,to the detector through a threaded insulated Abushing 5| cooperating with a complementarily threaded outer bushing 53 which supports the detector transversely across the waveguide I5. The output terminal arrangement of the control detector 31 is identical to that illustrated and described with respect to the signal detector I1.

,The control detector output terminal 55 is connected to any control circuit which regulates the 'local oscillator frequency in any manner known in the art.

It should be understood that one of the printector system is that received signals are applied separately and selectively to the two crystal de- 35 tectors and that the local oscillator signals also are applied separately and selectively to both crystals whereby undesirable reaction between input circuits and the local oscillator circuit, and

between the two detector circuits, is obviated.

Also due to the critical dimensions of the various sections of the receiver waveguide, loading of the local oscillator by means of the signal fcrystal or the T-R box is effectively prevented. AIt should be understood that the critical waveguide dimensions described herein for the purpose of illustration may be modiied in each in- .stance by adding thereto any desired number of Vhalf wavelengths, since such modiiication of the waveguide dimensions would have no effect other than that of possible reversal of signal phase, or `change of bandwidth.

Thus, the invention described comprises an improved method of and means for receiving and ,transmitting microwave pulse signals wherein the microwave receiver includes a tuned microwave crystal signal detector and a crystal automaticfrequency-control detector separately energized from the input signal source, and separately and selectively coupled to a local oscillator, for providing controlled intermediate frequency signals in a superheterodyne network. Critical dimerisions of the waveguide structure coupling the microwave crystals and their tuning networks eiectively prevents loading of the local oscillator and provides proper matching of circuit impedances.

I claim as my invention:

1. In a microwave signal receiver including a signal detector, means for connecting to a source of heterodyning signals and a second detector for deriving automatic frequency control potentials, the improvement comprising a waveguide having said heterodyne signal connecting means coupled to a portion thereof intermediate its ends, separateV tunable means coupled to said waveguide for coupling said'detectors from opposite Ydirections lthrough `said waveguide to :said heterodyne Asignal means, means for introducing signals 4,to `be Vreceived into both ends of said waveguide for separately applying said received signals to said `detectors and means for effectively isolating both of said signal introducing vmeans from said heterodyne signal connecting means.

2. In a microwave signal receiver including a signal detector, means for connecting to a source of `heterodyning signals and a second detector `for ,deriving automatic frequency control potentials, the improvement comprising a waveguide Ahaving said heterodyne signal connecting means coupled to a portion thereof intermediate its ends, separate tunable means coupled to said waveguide for coupling said detectors from opposite directions -through said waveguide to said Vheterodyne signal means, means for introducing signals into both ends of said waveguide for separately applying said signals to said detectors and means for effectively isolating both of said signal introducing means from said heterodyne signal Yconnecting means.

3. In a microwave signal receiver including a signal detector, means for connecting to a source kof heterodyning signals and a second detector for `deriving automatic frequency control potentials, the improvement comprising a waveguide having said heterodyne signal connecting means coupled to a portion thereof intermediate its ends, separate tunable means coupled to said waveguide for coupling said detectors from opposite directions through said waveguide to said heterodyn signal means, means for selectively introducing transmitted and reecived signals into opposite ends Vof said waveguide for separately applying said transmitted and received signals to diiferent ones ofsaid-detectors, and means for eiectively isolating both of said signal introducing means from said heterodyne signal .connecting means.

4. A microwave -superheterodyne detector network comprising a section of waveguide for input signals, a signal detector coupled to said waveguide, a selectively operable transmit-receive switching device interposed in said waveguide at a point in said waveguide intermediate one end thereof and said detector, tunable aperture means interposed in said waveguide intermediate said switching device and said detector coupling location at a point substantially preventing bypassing of said detector by said input signals and simultaneously substantially matching said detector to said waveguide, a control detector coupled to said waveguide for deriving from said input signals automatic-frequency-control potentials, means disposed intermediate said coupling locations of said detectors for coupling heterodyne Ysignals into said waveguide and to both of said detectors, separate means for controlling said heterodyne signal coupling to each of said detectors, means for introducingsaid input signals into opposite ends of said waveguide section, and means for controlling the coupling through said waveguide of said introduced signals to each of said detectors. v

5. A microwave superheterodyne detector network comprising a section of waveguide for input signals, a signal detector coupled to said waveguide, a selectively operable transmit-receive switching device interposed in said waveguide at fa point in said waveguide intermediate one end thereof and said detector, tunable aperture means interposed in said waveguide intermediate said switching device and saiddetector coupling location ata point substantiallyepreventing yby-passing of said detector by :said input signals and simultaneously substantially matching said detector impedance to said waveguide, a control detector coupled to said waveguide for yderiving from said input signals automatic-frequency-control potentials, means disposed intermediate said coupling locations of said detectors for coupling heterodyne signals into said waveguide and to both of said detectors, separate means for controlling said heterodyne signal coupling to each of said detectors, means for `introducing signals to be received into opposite ends of said waveguide section, and means for controlling the coupling through said waveguide of -said received signals to each of said detectors.

6. A microwave superheterodyne detector network comprising a section of waveguide for input signals, a signal detector rcoupled to said waveguide, a 'selectively -operable transmit-receive switching-device interposed in said waveguide at a point in said waveguide intermediate one end thereof and saiddetector, tunable aperture means interposed .in said waveguide intermediate said switching device-and saiddetector coupling location at a point-substantially preventing by-passing of said detector by Asaid `input signals and simultaneously substantially matching said detector impedance to said waveguide, a control detector coupled to said waveguide for deriving from said input signals automatic-frequencycontrol potentials,'means `disposed intermediate said coupling locations of said detectors for coupling heterodyne signals into said waveguide and to both of :said detectors, separate means for controlling said heterodyne signal coupling to each of said detectors, means for introducing transmitted and received signals into opposite ends of said waveguide section for separately coupling said :transmitted and received signals to different ones of said detectors, and means for controlling said coupling through said waveguide of said introduced signals to each of said detectors.

7. A microwave superheterodyne detector network comprising asection of waveguide for input signals, a signal detector coupled to said waveguide at a .point intermediate the ends of said waveguide, a yselectively operable transmitreceive switching-device interposed in said waveguide at a point spaced a distance of substantially one and three-eighths wavelengths at the operating microwave frequency from said coupling point of said detector, tunable aperture means interposed in said waveguide intermediate said switching deviceand saidcoupling point of said detector at a vpoini'fspaced substantially one-half wavelength from ,said detector coupling point for substantially preventing by-passing of said detector by` said input signals and for simultaneously substantially matching said detector to said waveguide, -a control detector coupled to said waveguide for yderiving from said signals automaticfrequencycontrol potentials, means disposed intermediate said coupling points of said detectors substantially equldistant Atherefrom for coupling heterodyne signals into said waveguide and to both of said detectors, separate means for controlling said heterodyne signal coupling to each of said detectors, means for introducing input signals into opposite ends of said waveguide section, and means for controlling the coupling through said Iwaveguide of said introduced sig- Ynals to `,each of said detectors.

8. A microwave superheterodyne detector net work comprising a section of waveguide for input signals, a signal detector coupled to said waveguide at a point intermediate the ends of said waveguide, a selectively operable transmit-receive switching device interposed in said waveguide at a point spaced a distance of substantially one and three-eighths wavelengths at the operating from said coupling point of said detector, tunable aperture means interstantially preventing by-passing of said detector by said input signals and for simultaneously substantially matching said detector to said waveguide, a control detector coupled to said waveguide for deriving from said signals automaticfrequency-control potentials, means coupling heterodyne signals into said waveguide and to both of said detectors, separate tunable 9. Apparatus according to claim 8 including a microwave attenuator interposed in said wavepreventng reaction from said control detector.

l1. A microwave superheterodyne detector network comprising a section of waveguide for input signals, a signal detector coupled to said waveguide at a point intermediate the ends of said waveguide, a selectively operable transmitreceive switching device interposed in said waveguide at a point spaced substantially one and three-eighths wavelengths at the operating microwave frequency from said coupling point of said detector, tunable aperture means interposed in said waveguide intermediate said switching device and said coupling point of said detector at a point spaced substantially .48 wavelength from said detector coupling point for substantially preventing by-passing of said detector by said input signals and for simultaneously substantially matching said detector to said waveguide, a control detector coupled to said waveguide for deriving from said signals automatic-frequencycontrol potentials, means disposed intermediate said coupling points of said detectors and spaced substantially equidistant therefrom for coupling heterodyne signals into said waveguide and to both of said detectors, separate tunable aperture means for controlling said heterodyne signal coupling to each of said detectors, said apertures being spaced substantially an odd number of quarter wavelengths from said detectors for substantially preventing loading of said detectors by said heterodyne signal coupling means for introducing input signals into opposite ends [gig-1 wavelengths where n is an integral number of quarter wavelengths at the operating microwave frequency from said coupling point of said detector, tunable aperture means interposed in said waveguide intermedia-te said switching device and said coupling point of said detecter at a point spaced substantially an integral number of half wavelengths from said detector coupling point for substantially preventing by-passing of said detector by said input signals and for simultaneously substantially matching said detector to said waveguide, a control detector coupled to said waveguide at a point spaced an odd number of half wavelengths from said signal detector for deriving from said signals automatic-frequency-control potentials, means disposed intermediate said coupling points of said detectors and spaced substantially equidistant therefrom for coupling heterodyne signals into said waveguide and to both of said detectors, separate tunable aperture means for controlling said heterodyne signal coupling to each of said detectors, means for introducing input signals into opposite ends of said waveguide section, and means for controlling the coupling through said wave guide of said introduced signals to each of said detectors.

13. Apparatus according to claim 8 wherein one of said input signal introducing means introduces a transverse electric eld component of said input signals into the end of said waveguide adjacent to said control detector.

14. In a microwave signal receiver including a signal detector, means for connecting to a, source of heterodyning signals and a second detector for deriving automatic frequency control potentials, the improvement comprising a waveguide having said heterodyne signal connecting means coupled to a portion thereof intermediate its ends, separate means coupled to said waveguide for coupling said detectors from opposite directions through said waveguide to said heterodyne signal means, means for introducing signals to be received into both ends of said waveguide for separately applying said received signals to said detectors and means for effectively isolating both of said signal introducing means from said heterodyne signal connecting means.

THOMAS M. GLUYAS, JR.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS 

